1. Field of the Invention
The present invention relates to a focusing system which can be incorporated in a single lens reflex camera, a lens shutter type camera, or a compact camera.
2. Description of the Related Art
In a passive autofocusing system for a single lens reflex camera, object light incident upon a photographing lens is condensed by a condenser lens and is split by a separator lens, so that the split beams are converged onto a pair of receiving areas on an array of light receiving elements to form object images. Thereafter, the distance between the object images formed on each receiving area is detected in accordance with the integral values output from the light receiving element array. An amount of defocus (defocus amount) is obtained in accordance with the distance between the object images. The autofocusing system moves a focusing lens group of the photographing lens, so that the defocus amount is zero, i.e., the distance between the object images formed on each light receiving area is zero.
In a passive autofocusing system for a lens shutter type camera or a compact camera, an object image is formed onto a pair of light receiving element arrays by a pair of imaging lenses through a photographing lens. The distance between the object images formed on each receiving element array is detected in accordance with integration values of the output from the light receiving element arrays. Consequently, the object distance is measured by triangulation, based on the base length of a pair of imaging lenses and the f-number of the imaging lenses. The autofocusing system moves a focusing lens group of the photographing lens in accordance with the object distance. A CCD line sensor or a MOS line sensor can be used as the light receiving element array.
In particular, in an autofocusing system in a lens shutter type camera, the object distance is measured by a triangulation. Hence, as the base length, i.e., the distance between the imaging lenses increases, the measurement precision becomes high. Moreover, in general, since the imaging lens has a fixed focus, the object image formed on the light receiving element array is dimmed depending on the object distance. The dimmed image deteriorates the precision in the measurement of the position of the object image. To reduce a measurement error, it is preferable that the distance between a pair of light receiving element arrays be increased.
In a single lens reflex camera, the position adjustment of the focusing lens is accomplished by repeatedly effecting the integrating operations of the CCD line sensor to minimize the defocus amount. Therefore, it is possible to obtain a high measurement precision without increasing the distance between the object images in comparison with the lens shutter type camera. Furthermore, in a single lens reflex camera, since an AF unit including a condenser lens, a separator lens, a CCD line sensor, etc., is housed in a lower portion of a mirror box, not only must the AF system be miniaturized, but also the size and pitch of the light receiving elements of the CCD must be reduced.
In an autofocusing system using a CCD line sensor, the integration time of the light receiving elements is controlled in accordance with an output of a monitor provided adjacent to the light receiving element array to prevent the electric charges integrated, i.e., accumulated, in the light receiving elements which have received the object light from overflowing. To detect the defocus amount, a pair of object images are formed on one light receiving element array and the distance between the object images is detected. Therefore, it is necessary for a monitor sensor to monitor only half the light receiving area of the light receiving element array. However, in the case of the lens shutter type camera in which the distance between the object images formed on each of the spaced light receiving element arrays is measured to detect the object distance, the monitor sensor must monitor the substantially entire light receiving area of at least one of the light receiving element arrays.
Therefore, in the prior art, different autofocusing systems having different optical systems and light receiving element arrays must be prepared for the single lens reflex camera and the lens shutter type camera.